Curtains, blinds or other window coverings can also affect the performance of sprinklers. When these materials are placed between the glass and sprinkler, the water is unable to cool the glass, thus causing the glass to fail early on during a fire. Some sprinkler manufacturers advise that blinds or curtains must not be placed between sprinklers and glass.

Locating sprinkler heads up to 12 inches (30 cm) away from the windows presents an awkward challenge. Once tenant improvements are made to a building, landlords and owners may find that initial advisories regarding window coverings may be difficult to enforce.

Two additional conditions are worth mentioning. Since the source of potential fires cannot always be identified ahead of time, the sprinkler and tempered glass systems require deluge sprinklers on both sides of the glass. Also, window designs with intermediate horizontal mullions are not allowed, since the mullions would interfere with the water's ability to evenly bathe the glass surface.

Other laboratory tests conducted over the last several years demonstrate the complexity of the sprinkler and glass relationship. In 1995, officials at Factory Mutual Research Corporation observed a test to determine if a non-fire-rated window assembly and sprinkler system could provide equal protection to that of a fire-rated assembly. The system (which combined specially designed sprinklers and tempered or heat-strengthened glass) was exposed to fire, with the hope that the glass would be able to stay intact.

The researchers discovered that the glass could survive the test if two conditions were met. First, the fire had to start far away from the glazing assembly, in this case approximately 8 feet (2.44 m). Second, the sprinkler needed to activate soon after the fire started.

When the heat source was brought closer to the non-fire-rated glass, the glass fell out of the frame in less than five minutes. Apparently, the close proximity of the flames caused the temperature of the glass to rise too quickly, outpacing the sprinklers' ability to cool the glass surface.

Nearly a decade prior to that test, Lawrence Livermore National Laboratory (LLNL) conducted a similar experiment to find out what would happen when a fire started near the surface of non-fire-rated, tempered glass (see Fire Journal, Vol. 80, No. 4). They used two different sizes of fire, 250 kiloWatt (kW) and 40 kW, with sprinklers installed for the test. When the larger fire was started, the sprinklers activated early and the glass remained intact. But in both tests conducted with the smaller fire, the glass fractured and fell out of the test assemblies in less than four minutes, even before the sprinklers activated.

Looking at the results of both tests, it appears that when fires are relatively large and distant, non-rated, tempered glass may perform as needed. The overall room temperature rises rapidly enough to activate sprinklers before the glass becomes stressed. However, when a smaller fire is concentrated close to the glass surface, it may not activate sprinklers early enough, generating sufficient stress to shatter the non-rated glass.

Clearly, sprinklers have improved the standard of fire safety. Yet, as the NFPA has acknowledged in their evaluation of U.S. experience with sprinklers, sprinklers are not adequate alone; they must be part of a comprehensive fire protection program. If, for any reason, a sprinkler system should fail, there must be some form of compartmentation in place to prevent the fire from spreading further.

There are many glass products available that offer outstanding fire ratings and the ability to withstand thermal shock. From a life safety standpoint, there is no reason to compromise on either issue. Combining sprinklers with fire-rated glazing offers the best of both worlds.

Summary

The benefits of fire-rated glazing include safety, performance, flexibility, and aesthetics. Developments in fire-rated glass, related technology, and framing systems offer architects many planning and design options that simultaneously meet design goals and life safety code requirements.

Jerry Razwick is president of Technical Glass Products. He frequently writes and speaks nationwide on fire-rated glass and framing.